1. Field of the Invention
The present invention is directed to starch-oil type sizing compositions for glass fibers which provide improved tensile strength and heat cleaning properties in glass fibers treated with a composition.
2. Technical Considerations and Prior Art
Glass fibers are typically coated after forming with a sizing composition that imparts desired properties as discussed below. As used herein, the term "size", "sized", "sizing" means a coating composition applied to the glass fibers immediately after formation of the fibers. The sizing composition functions as a lubricant and binding agent to protect the fibers from abrasion with each other, increase the tensile strength of the strand and improve the performance of the fibers during further processing, such as but not limited to weaving of the fibers to produce a fabric. The sizing is also used to impart to fabrics that are woven from treated fibers a surface that is smooth and has few visible defects.
Although not required, the size applied to glass fibers that are typically used in a weaving operation is a "starch-oil sizing", which is a term commonly used in the art and as used herein means a treatment for fibers having at least one silane, at least one starch and at least one lubricant which is applied to the glass fibers to impart desired properties. Typically, the starch-oil size applied to textile fibers is removed after weaving so as to develop maximum integration between protective color coatings or polymers subsequently applied to the fabric and the glass fiber surfaces. The fibers can be cleaned by a water wash or by solvents but it is preferred to burn the size from the glass fiber surfaces. The heat cleaning operation can be accomplished in a single or multiple steps in a manner well known to those skilled in the art. For example and without limiting the present invention, in a single-step operation, the woven fabric is passed through a radiant oven and exposed to temperatures of between about 538.degree. C. to about 677.degree. C. (about 1000.degree. F. to about 1250.degree. F.) for a period long enough to volatilize and burn the organic constituents of the sizing composition. In multi-stage operations, the fabric is exposed to high temperatures for a shorter period of time and collected on a roll. The roll is then placed in a second oven and exposed to lower elevated temperatures for a sufficient period of time to heat the fabric and volatilize the remaining organic sizing constituents. Typically, the heat-cleaning process reduces the loss on ignition to about 0.1 to about 0.4. In applications that incorporate the fabric into laminates for supports used in the electronics industry, such as but not limited to printed circuit boards, the loss on ignition can be as low as about 0.05 or less.
It has been found that the dramatic thermal treatment of the glass fibers during a heat-cleaning process can reduce the glass fiber tensile strength by as much as about 70%. In addition, the heat cleaning presents the continual problem of leaving undesired colored residue deposits on the glass fiber fabric.
U.S. Pat. Nos. 5,120,363; 5,334,639 and 5,393,335 disclose aqueous starch-oil sizing compositions, which improve processability in woven and non-woven applications. The sizing combines starch, lubricants and silane coupling agents, emulsifiers, defoamers and biocides can also be included in the sizing composition.
WO 94/11318 discloses sizing compositions, which include bis-silanes useful for glass fiber reinforcements in the area of filament winding and pultrusion.
U.S. Pat. No. 3,502,456 discloses a process and apparatus for heat cleaning glass fiber fabric. In order to remove the sizing composition, the sized fabric is moved continuously through a cleaning chamber, which applies size-volatilizing heat to the glass fiber fabric, by the convection action of gases generated by combustion of a pressure-fed fuel.
It would be advantageous to provide a sizing composition, which can be removed by a heat cleaning process while maintaining a high tensile strength in the glass fibers and reducing the possibility of leaving colored residue deposits on the heat cleaned glass fiber fabric.